drm/amdgpu: add a custom GTT memory manager v2

Only allocate address space when we really need it.

v2: fix a typo, add correct function description,
    stop leaking the node in the error case.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>